User blog comment:LegionZero/Nero's Parallels to Vergil and Sparda/@comment-11462520-20140910173627/@comment-6769189-20140910223029

Honestly I don't know. There are defintely many, many possibilities that would make sense within the DMCverse:

1) Something similar to Project S/Project G from Final Fantasy VII. Having Nero, while in the womb, be infused with something(blood/energy/essence) of Sparda, Vergil, or Dante.

2) Something like how Greed-Ling and Wrath were made in Fullmetal Alchemist: Brotherhood/Pride in Fullmetal Alchemist. Where Nero is injected with Sparda, Vergil, or Dante's blood/aura/power/essence right into his bloodstream as a baby/fetus/newborn and it changes him and his genes/blood genetics.

3) something like this thing i came up with

most of my theories have to do with an infusion/infection of some kind or an outright copy/clone/spirtual copy. One conclusion i have come up with as a definite, Nero can't be Vergil's son by way of Vergil putting his wiener in a pretty lady.